Field of the Invention
The invention relates generally to offshore structures to facilitate offshore oil and gas drilling and production operations. More particularly, the invention relates to compliant offshore towers releasably secured to the sea floor.
Background of the Technology
Various types of offshore structures may be employed to drill and/or produce subsea oil and gas wells. Usually, the type of offshore structure selected for a particular application will depend on the depth of water at the well location. For water depths up to about 600 ft., fixed platforms are often employed. Fixed platforms include a concrete and/or steel jacket anchored directly to the sea floor, and a deck positioned above the sea surface and mounted to the upper end of the jacket.
Fabrication and installation of a fixed platform requires a particular infrastructure and skilled labor. For example, launch barges are needed to transport the components of the jacket and the deck to the offshore installation site, derrick barges are needed to position and lift the upper portion of the jacket, and derrick barges are needed to lift and position the deck atop the jacket. In addition, installation of a fixed platform often requires the installation of piles that are driven into the seabed to anchor the jacket thereto. In deeper applications, additional skirt piles must also be driven into the seabed. In select geographic locations such as the Gulf of Mexico, fixed jacket platforms are fabricated, deployed, and installed on a regular basis. Accordingly, such regions typically have the experience, infrastructure, and skilled labor to enable fixed jacket platforms to provide a viable, competitive option for offshore drilling and/or production. In other regions, having little to no experience with fixed jacket platforms, the facilities, equipment, infrastructure, and labor may be insufficient to efficiently construct, deploy, and install a fixed jacket platform. Moreover, even in some regions, such as Brazil and Peru, that have some experience fabricating and installing fixed jacket platforms, the range of applications for fixed jacket platforms anticipated in the next few years may exceed present capabilities.
Fixed jacket platform are typically designed to have a natural period that is less than any appreciable, wave energy anticipated at the offshore installation site. This is relatively easy to accomplish in shallow waters. However, as water depths increase, the inherent compliance, and hence natural period, of the jacket increases. To reduce the natural period of the jacket below the anticipated wave energy as water depth increases, the jacket is stiffened by increasing the size and strength of the jacket legs and pilings. Such changes may further increase the infrastructure and labor requirements for fabrication and installation of the jacket.
Compliant towers offer another alternative for offshore applications with water depths up to about 600 ft. Compliant towers include a truss structure anchored directly to the sea floor, and a deck positioned above the sea surface and mounted to the upper end of the truss structure. Although the lower end of the truss structure is rigidly secured to the sea floor, the truss structure is designed to flex over its length in response to environmental loads. However, the lower end of the truss structure is typically secured to the sea floor with piles that are driven into the sea bed, and thus, provides some of the same installation challenges as fixed jacket platforms.
Accordingly, there remains a need in the art for offshore drilling and/or production bottom-founded structures anchored to the sea floor that require less infrastructure and specialized labor to fabricate and install. Such offshore systems would be particularly well-received if they could be transported offshore and between different installation sites with relative ease.